The present invention relates to an image reading apparatus such as a facsimile or copying machine, etc. and more particularly to a character and picture information reading apparatus in which the document such as a book is fixedly placed on a document glass plate, and the document glass plate or an optical scan system is scanned to read image information.
With the size of document being optional in the image reading apparatus of this type, a substantial waste will be caused unless an image reading range is limited only to an area where the document is present. In a facsimile machine, for example, if an area of A4 size is read with respect to the document of B5 size, information wasteful in about 1/3 portion thereof would be transmitted and, therefore, a transmission time would be prolonged about 1.5 times longer than the normal case. In a machine where the reading size is selectable using a switch, such a waste in reading, etc. can be avoided by an operator to make proper switch operation. But erroneous operation may cause missing information, wasteful reading, etc. Moreover, if the document is not properly positioned even with the document size being properly specified, correspondence between the document surface and the reading surface would be shifted to cause missing information, etc.
To avoid such disadvantages, it is envisaged to automatically detect the position and size of the document. The technique of this type is well known in usual facsimile machines which are designed to perform document reading while moving the sheet-like document, but it is hardly practiced because of various problems in machines of the type that the book-like document is placed on a document glass plate and an optical scan system or the document glass plate is scanned.
As to copying machines adapted to digitally process signals, there is known an apparatus that the entire process signals, there is known an apparatus that the entire surface of a pressure plate for pushing the document from the back side is formed into a reflection mirror, and the document is previously scanned once prior to reproduction processing so as to read the size and position of the document based on an image sensor output signal resulted from that scan. According to this apparatus, since the scan range to be read is automatically set, erroneous operation will not be caused. In this case, however, one reproduction processing requires twice scans, which results in a disadvantage of prolonging the processing time.